Nuchal Transclucency Scan ( NT Scan)

The NT scan or "Nuchal Translucency" scan is a specialized scan to assess the risk for fetal chromosomal abnormalities in addition to the growth and development of the fetus. This is done between 11 -14 weeks of pregnancy. 

This scan as part of the first trimester screening for Down's syndrome and as an early anomaly assessment to all our patients routinely. When a baby is conceived, it inherits genetic material from its parents. The genetic material is usually transferred to the baby in the form of 46 chromosomes-23 from each parent. Therefore there are 46 chromosomes (arranged in 23 pairs) in each normal human cell. Most babies develop normally, but unfortunately a small number have significant problems. These problems may be due to structural defects or abnormalities in chromosomes and genes. Many structural defects can be detected by scanning but chromosomes and genes cannot be checked by scanning alone. The scan forms a "screening method" to detect certain "markers" that place a fetus at "low risk" or "high risk" for Down's Syndrome.

What is Down's Syndrome?

There are a number of chromosomal abnormalities, but Down's syndrome or Trisomy 21 is the most common. There is an extra chromosome of number 21 and hence the name Trisomy 21. It causes severe learning disability in children. There is no cure for Down's syndrome.

How common is Down's syndrome?

People do not usually expect to have a baby with Down's syndrome. It does not usually run in families. Some people think that only older women can have a baby with Down's syndrome, but this is not true. Anyone can have a baby with Down's syndrome, but the risk does go up with age. The older a mother is the more chance she has of having a baby with the condition. For example, the chance of having a baby with Down's syndrome is one in 1500 for women who are 20 years old, one in 900 for women who are 30 years old, and one in 100 for who are 40 years. However, all pregnant women are at risk of carrying a baby affected by Down's syndrome.

What can you do to find out whether or not your baby has Down's syndrome?

There are two different types of tests that can be done during pregnancy to look for Down's syndrome - screening tests and diagnostic tests.A screening test is offered to all women during early pregnancy to look at the chance in this pregnancy of the baby being born with Down's syndrome. It is important to understand that the screening test does not give a definite 'yes' or 'no' answer as to whether or not the baby does have Down's syndrome. If your screening test shows a high chance that the baby has Down's syndrome, you will usually be offered a diagnostic test.

What is first trimester screening?

This is a screening test done in the first trimester (before 14 weeks of pregnancy). It is a combination of fetal ultrasound and maternal blood tests and is called combined screening test.

Ultrasound: This ultrasound scan is called Nuchal Translucency scan (NT Scan). Nuchal Translucency is the name for the fluid behind the neck of a fetus. An NT scan uses ultrasound to measure the amount of fluid. The fluid under the skin at the back of the baby's neck appears "black" on ultrasound images and is normally present in all fetuses in early pregnancy. A fetus at risk of Down's syndrome tends to have a higher amount of fluid. The thickness of the fluid can predict whether the early fetus may have Down syndrome or a number of other chromosomal abnormalities. It is essential that the Nuchal Scan is performed between the 11th and 14th week of pregnancy, because the accuracy is best in this period.

Maternal Blood Test: On the day the scan is done a blood sample is taken from you. This blood test measures two substances (PAPP-A and B-hCG), which are present in blood of all pregnant woman but abnormal levels are associated with enhanced risk of chromosomal problems in the fetus.

We advise women to take the blood test between 10-11 weeks of pregnancy so that by the time they come for the NT scan, the blood test results are available and the combined test report can be generated immediately following the scan.

Calculating Risk: These two test reports are entered into a computer program along with your age to calculate an overall "risk" or ratio of your baby having Down's syndrome. The calculated risk assessment is used to guide the discussion between you and your doctor about any further investigation or treatment. When used in combination (Combined screening test) has a 90% ability to predict the risk for chromosomal problems, the most common of which is Down's syndrome, but it also used for other abnormalities like trisomy 18, 13, etc.

When will the report be available?

The results of combined screening test will be available after the NT Scan in case the blood samples for the first trimester double marker have been given earlier. For women who choose to have the blood test on the day of the scan, the final combined test report will be ready in a week.

Are there any risks with this test?

This test presents no risk to your baby. The main benefit is that because it is done in early pregnancy you can use the information to make an informed choice.

What happens if the test indicates a high risk (high chance) of chromosomal problem?

It is important to remember that this FTS will not diagnose Down's syndrome or other abnormalities. It only gives a risk assessment. You have to understand that if the risk is high, the only way to confirm the diagnosis is by testing for fetal chromosomes. These tests are chorionic villus sampling (CVS) and amniocentesis.

Chorionic villus sampling (CVS): Chorionic villus sampling (CVS) involves having a small sample of the placenta (the organ in which the fetus grows and is protected and nourished) taken for examination. It can be performed after 10 weeks of pregnancy. The sample is taken either by passing a thin needle through the wall of your abdomen. The needle, or tube, is put into the correct position using an ultrasound scan for guidance. The sample is taken and is later sent to a laboratory for testing. The results will be able to show whether your baby has Down's syndrome. Possible complications of CVS can include infection and heavy bleeding. Approximately 1% of women who have CVS also go on to experience a miscarriage. However, it is important to remember that the risk of miscarriage is very small, and most procedures cause no complications.

Amniocentesis: Amniocentesis involves taking a small sample of amniotic fluid (the fluid that surrounds the fetus in the womb) for examination. It is usually performed after 15 weeks and can be carried out until 22 weeks. The sample is taken by passing a needle through your abdomen and womb. The amniotic fluid can then be drawn out through a syringe. As with CVS; the needle is guided into the correct position using an ultrasound scan. After the sample of amniotic fluid has been taken, it is sent to a laboratory for testing. Possible complications of amniocentesis can include infection, or injury, to you, or your baby. As with amniocentesis, there is also a 1% chance of miscarriage. Amniocentesis is a fairly common procedure, and the risk of complications is low.

What are the other advantages of doing this test?

Other things that can be done at the time of the scan are:

Determine your expected date of delivery

Check that the baby is developing normally Diagnose twins

It also tests blood flow to the uterus (uterine artery Doppler), which predicts adverse pregnancy outcomes like preeclampsia (high BP in mother) and fetal growth restriction

What if you miss the tests i.e. come after 14 weeks of pregnancy?

If you are more than 14 weeks pregnant, you will not be offered the combined screening test. It is not possible to do it. Instead you will be offered Quadruple test. This is offered between 15-19 weeks of pregnancy. Beyond 20 weeks there are no interpretable screening tests. This test looks at different substances and doesn't include NT scan. Like a combined test, the computer program uses the results from blood test, scan and mothers details and work out a risk figure. The sensitivity of this test is 60-70%

The report is generally followed by a declaration of the limitations of the scan and an undertaking that while conducting antenatal scans, WE DO NOT REVEAL THE SEX OF THE FETUS TO ANYBODY IN ANY MANNER.

Please also check the dates allotted for the follow-up scan/counseling session and book your appointments well in advance for the same.